1. Field of the Invention
The present invention relates to enrichment and separation of isotopes, in particular .sup.14 N and .sup.15 N, and .sup.12 C and .sup.13 C by infrared radiation induced isomerization of isocyanides.
2. Description of the Prior Art
The projected use of .sup.15 N in the nuclear industry is expected to reach 300,000 kg/year. Use is in liquid-metal fast breeder reactors. Also, .sup.15 N and .sup.13 C are presently used as tracers in the fields of medicine and research. Low cost schemes for separating these isotopes are accordingly required.
Ambartsumyan et al. in Vol. 17, Journal of Experimental and Theoretical Physics Letters, pp. 63-65 (1973) disclose isotope separation of .sup.14 N and .sup.15 N by a two-step photodissociation of .sup.14 NH.sub.3 and .sup.15 NH.sub.3, in which monochromatic radiation of a frequency .nu..sub.1 selectively excites a vibrational transition of molecules of only one isotopic composition. The molecules are simultaneously illuminated with light of frequency .nu..sub.2, the quantum energy of which is sufficient for photodissociation of only the vibrationally excited molecules. However, this is a costly process and provides a possibility of isotopic scrambling due to four different intermediate chemical reactions.
Brauman et al. in Optics Communications, Vol. 12, No. 2, pp. 223-224 (1974) propose isotope separation by selective unimolecular photoisomerization. The proposed procedure comprises placing isomer A, having a mixed isotopic composition, in a reaction chamber, where it is excited in an isotopically selective manner. Some of the excited A is deactivated back to the ground state, while the rest is converted to isomer B, which is now enriched in the isotope of interest. The mixture is then removed from the reaction vessel and the isomers are separated by conventional means, such as by distillation or by chromatography. The desired isotope can then be removed from isomer B by carefully designed physical or chemical methods. The authors suggest possible reactions, which include cis, trans isomerizations (e.g., dihaloethylenes) via singlets or triplets, valence bond isomerizations (e.g., dienes to cyclobutanes) and others (e.g., isocyanides to cyanides). However, other than suggesting the foregoing general reaction schemes, there are no details as to proposed reaction mechanisms, laser wavelengths that must be employed or other experimental details.
Robinson et al. in U.S. Pat. No. 4,049,515, issued Sept. 20, 1977, disclose laser isotope separation schemes by multiple photon absorption. Briefly, the schemes involve irradiating a molecular species having at least two isotopes of an element with infrared laser light of a frequency which selectively excites to a vibrational level only those molecules of the molecular species containing a particular isotope. Use of multiple photon absorption produces a sufficiently energetic vibrational state such that the molecules containing the particular isotope undergo a chemical reaction, such as dissociation or reaction with a second molecular species. The patent discloses two examples for which laser induced enrichment was obtained; namely the enrichment of .sup.34 S in natural SF.sub.6 and .sup.11 B in natural BCl.sub.3. However, there is no teaching therein of a method for selecting a candidate molecular species from the essentially infinite number of species which exist for any given element for which the process of Robinson et al. is applicable. When a molecule is subjected to infrared radiation in the manner taught by Robinson et al., the isotopic shift is generally masked by other vibrational modes or, if unmasked, has a magnitude lower than that required for isotope separation. In addition, it has not been possible to precisely predict the manner in which a molecule will dissociate or react with other species when subjected to high intensity infrared radiation. Hence no general method can be given for devising a laser-chemical reaction system which will effect removal of the desired isotopic species. As a result, it has heretofore not been possible to predict which molecules lend themselves to laser isotope separation by multiple photon absorption.
Although present usage of .sup.15 N is small, projected use in core elements of liquid-metal fast breeder reactors is considerable, as mentioned above. Current separation of .sup.15 N is accomplished by NO distillation, or by chemical exchange between NO and HNO.sub.3. The latter process has an enrichment factor of about 1.055. The only proposed separation of nitrogen isotopes with a laser discussed above (Ambartsumyan et al.) suggests an isotopic enrichment factor of about 4. However, the considerable potential for isotopic scrambling renders the proposed process unsuitable on a commercial scale.
Carbon isotopes are presently separated by low temperature distillation of carbon monoxide and by gas phase thermal diffusion of methane. The carbon monoxide process for separating .sup.13 C is based on a vapor pressure differential that yields an enrichment factor of about 1.011.